Continuously variable transmissions (CVT's) are used almost universally in snowmobiles to alleviate any need for the driver to shift the transmission as the vehicle accelerates through its range of speeds. The assignee of the present invention has also successfully utilized CVT's in its line of ATV's. Typically the CVT transmission is connected to the output shaft of the vehicle's engine, the transmission providing continuously variable gear reduction from the relatively higher rotation speed of the engine to the relatively lower rotation speed of the vehicle drive axle. A CVT may be used in conjunction with an additional gear box/transmission, if desired. For example, on ATV's it is desirable to provide a gear box to permit the driver to shift between forward and reverse gears. In such cases a neutral position may also be provided, along with, e.g., an optional low gear for extra power at low speeds. Typically such a gear box is connected to the output shaft of the CVT, the gear box in turn having an output connected by suitable linkages to the drive axle (or axles, in a four or six wheel drive vehicle) of the vehicle. A gear box or transmission may also be utilized for further gear reduction in addition to the reduction provided by the CVT, and other standard drive train components (such as differentials, etc.) may also be utilized between the CVT and the drive axle(s).
Typically a CVT transmission is comprised of a split sheave primary drive clutch connected to the output of the vehicle engine and a split sheave secondary clutch connected (often through additional drive train linkages) to the vehicle axle. An endless, flexible, generally V-shaped drive belt is disposed about the clutches. Each of the clutches has a pair of complementary sheaves, one of the sheaves being laterally movable with respect to the other. The effective gear ratio of the transmission is determined by the positions of the movable sheaves in each of the clutches. The primary drive clutch has its sheaves normally biased apart (e.g., by a coil spring), so that when the engine is at idle speeds the drive belt does not effectively engage the sheaves, thereby conveying essentially no driving force to the secondary driven clutch. The secondary driven clutch has its sheaves normally biased together (e.g., by a torsion spring working in combination with a helix-type cam, as described below), so that when the engine is at idle speeds the drive belt rides near the outer perimeter of the driven clutch sheaves.
The spacing of the sheaves in the primary drive clutch usually is controlled by centrifugal flyweights. As the drive clutch rotates faster (in response to increased engine rpm) the flyweights urge the movable sheave toward the stationary sheave. This pinches the drive belt, causing the belt to begin rotating with the drive clutch, the belt in turn causing the driven clutch to begin to rotate. Further movement of the drive clutch's movable sheave toward the stationary sheave forces the belt to climb outwardly on the drive clutch sheaves, increasing the effective diameter of the drive belt path around the drive clutch. Thus, the spacing of the sheaves in the drive clutch changes based on engine rpm. The clutch therefore can be said to be speed sensitive.
As the sheaves of the drive clutch pinch the drive belt and force the belt to climb outwardly on the drive clutch sheaves, the belt (not being stretchable) is pulled inwardly between the sheaves of the driven clutch, decreasing the effective diameter of the drive belt path around the driven clutch. This movement of the belt inwardly and outwardly on the drive and driven clutches smoothly changes the effective gear ratio of the transmission in infinitely variable increments. (CVT's hence are sometimes referred to as infinitely variable transmissions).
The spacing of the sheaves in the driven clutch usually is controlled by a different mechanism. Although a coil spring could be used to bias the sheaves of the driven clutch together, typically a more sophisticated torque-sensitive system is used to pinch the belt harder as more torque is conveyed by the drive belt to the driven clutch. A generally cylindrical cam with, e.g., three cam surfaces (often called ramps) on one end is secured to the output shaft of the driven clutch. Because the ramps are generally helical in shape, this cam is often referred to a helix. A set of a corresponding number of cam followers--typically buttons or rollers--is mounted to the movable sheave, and the movable sheave is mounted within the driven clutch so that it is free to move laterally and is also rotatable with respect to the shaft. The buttons or rollers are mounted in positions aligned with the ramps of the helix, and a torsion spring usually is used to urge the movable sheave rotationally to keep the buttons or rollers engaged against their respective helix ramps.
As torque is transmitted by the drive belt to the driven clutch sheaves, the belt tends to urge the moveable sheave laterally away from the stationary sheave, and also tends to rotate the movable sheave with respect to the shaft. Since the buttons are held against the ramps by the torsion spring, the torque being applied by the belt to the movable sheave tends to cause the buttons to slide up the ramps, which in turn tends to push the movable sheave toward the stationary sheave. Thus, the helix converts the torque of the drive belt to a force that pinches the sheaves together, providing good frictional contact of the sheaves against the drive belt. The more torque applied by the belt to the driven clutch, the harder the sheaves of the driven clutch pinch the belt, thereby preventing the belt from slipping and also tending to cause the transmission to downshift (i.e., urging the belt outwardly between the sheaves of the driven clutch, which causes the belt to move inwardly between the sheaves of the drive clutch). Thus, the spacing of the sheaves in the driven clutch changes based on torque. The clutch therefore can be said to be torque sensitive.
The actual position of the belt within the sheaves of the drive and driven clutches is determined by the balance of the forces acting on the moveable sheaves in the two clutches. In the drive clutch, these forces consist of the coil spring urging the sheaves apart and the speed-dependent force of the centrifugal flyweights which urges the sheaves together. In the driven clutch, these forces consist of the coil spring urging the sheaves together along with the torque-dependent force generated by the rollers on the helix ramps.
As mentioned above, the position of the drive belt between the clutch sheaves is determined by the balance of forces acting on the movable sheaves. In variable operating conditions this balance can be disrupted. For example, when the vehicle is traveling along at a given speed and then the rider momentarily lets off on the throttle, the balance of forces is disrupted, and can cause the system to momentarily shift out of the desired ratio. When the rider then again applies the throttle, torque is restored to the driven clutch, but the transmission is no longer in its optimal gear ratio, and takes a moment to adjust. Similarly, if the drive wheels momentarily leave the ground (such as when a professional rider goes off a jump) but the rider does not let off on the throttle, the load on the drive wheels is momentarily substantially reduced, again disrupting the balance of forces within the CVT and causing it to temporarily shift out of the desired gear ratio. When load is restored to the drive wheels, the CVT must again readjust to the proper gear ratio.
When the CVT must quickly downshift or upshift to return to the proper gear ratio, the belt must move outwardly or inwardly between the sheaves of the driven clutch, a movement that can be inhibited by the need of the moveable sheave to rotate with respect to the stationary sheave as the rollers travel along the helix ramp. This rotation of one sheave with respect to the other causes scrub on the sides of the drive belt, and, thus, frictional forces can prevent this shifting from happening as smoothly and quickly as would be desirable.
Due at least in part to this dynamic function of the CVT, the CVT does not provide significant engine braking through backdriving the engine. That is, in some types of vehicle drive trains when the vehicle is traveling along at a given speed and then the throttle is dropped (e.g., to an idle speed), the rotation of the drive wheels of the vehicle will backdrive the drive train, causing the engine to rotate at a speed greater than it otherwise would (based on throttle position). The inherent frictional forces present throughout the drive train, including particularly the compression forces present in the engine cylinders, tend to slow the vehicle down. This condition is commonly referred to as engine braking, and can be a useful feature. The degree of engine braking provided (in vehicles capable of doing so) is dependent on the gear ratio of the transmission-in higher gears less braking is provided, and in lower gears more braking is provided.
In a CVT, loss of balance of forces between the drive and driven clutches when the rider lets off on the throttle (including in particular the loss of the torque-induced pinching force by the helix on the belt) makes the CVT less effective in braking the engine. CVT systems also do not provide engine braking when the engine speed is reduced all the way to idle. That is, when the engine is simply idling, the drive clutch has its sheaves biased apart by a coil spring so that the sheaves do not effectively engage the drive belt. Usually the length of the drive belt is chosen so that it is a little bit loose in the idle position, preventing the shaft of the drive clutch from imparting any rotation to the drive belt and, thus, preventing the vehicle from "creeping". A consequence of the looseness of the drive belt, however, is that the driven clutch is not capable of backdriving the drive clutch (and, therefore, the engine) when the belt and clutches are in the idle position.